The present invention relates to networks having an electrical resistance changing with the magnitude of a sensed parameter and, more particularly, to novel interchangeable, linearized sensor networks using non-linear sensors and for methods for producing such networks.
Sensors which provide, between a pair of sensor terminals, an electrical resistance of magnitude changing with the magnitude of a sensed parameter, are well known. In many of these sensors a highly non-linear relationship exists between the magnitude of the sensor resistance and the sensed parameter magnitude, whereby relatively expensive linearizing networks are not only required between the sensor network and a linear display, but, if the sensor is to provide a linear output, the sensor must provide a slope and a specific value of resistance at a particular sensed parameter magnitude, which characteristics are exceedingly difficult and expensive to obtain. Lack of these output characteristics results in a low degree of interchangeability between different sensor networks of the same type.
In particular, it is well known that of various temperature sensing means, a potentially low-cost temperature sensor may be fabricated from that group of thermally-sensitive semiconductors collectively known as thermistors. A thermistor, usually manufactured from oxides of the iron group of transition elements (such as chromium, magnesium, iron, cobalt, nickel and uranium) which has been doped with ions of differing valance, classically exhibits an exponentially decreasing resistance with increasing temperature. The resulting large negative temperature coefficient (NTC) makes a thermistor an extremely attractive device for use as a temperature sensor. However, the extreme non-linearity of the exponential function complicates applications of the device. Positive temperature coefficient (PTC) thermistors are also known, which are more nearly linear, but have greatly reduced temperature coefficients compared to the NTC thermistors.
Thermistors and other sensors providing a terminal resistance varying with the magnitude of a sensed parameter, are particularly desirable for applications which require sensors which are not only low cost but also highly interchangeable. Interchangeability requires that the sensor network have: a particular value of terminal resistance (i.e. point resistance) for a selected value of sensed parameter magnitude; a desired slope in the resistance-versus-sensed parameter relationship; and a high degree of linearity. Thus, any one of a multiplicity of sensors of similar design can be interchanged with any other one of that group of sensors and still provide a substantial degree of accuracy without additional adjustment.
Accordingly, networks having an electrical resistance changing with the magnitude of a sensed parameter in a highly linear manner and with slope and point resistance easily established, are highly desirable.